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X-Rays from RU Lupi: Accretion and Winds in Classical T Tauri Stars

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X-Rays from RU Lupi: Accretion and Winds in Classical T Tauri Stars A&A 473, 229–238 (2007) Astronomy DOI: 10.1051/0004-6361:20077644 & c ESO 2007 Astrophysics X-rays from RU Lupi: accretion and winds in classical T Tauri stars J. Robrade and J. H. M. M. Schmitt Universität Hamburg, Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany e-mail: [email protected] Received 13 April 2007 / Accepted 18 June 2007 ABSTRACT Context. Low-mass stars are known to exhibit strong X-ray emission during their early evolutionary stages. This also applies to classical T Tauri stars (CTTS), whose X-ray emission differs from that of main-sequence stars in a number of aspects. Aims. We study the specific case of RU Lup, a well known accreting and wind-driving CTTS. In comparison with other bright CTTS we study possible signatures of accretion and winds in their X-ray emission. Methods. Using three XMM-Newton observations of RU Lup, we investigate its X-ray properties and their generating mechanisms. High-resolution X-ray spectra of RU Lup and other CTTS are compared to main-sequence stars. We examine the presence of a cool plasma excess and enhanced plasma density in relation to X-rays from accretion shocks and investigate anomalous strong X-ray absorption and its connection to winds or circumstellar material. Results. We find three distinguishable levels of activity among the observations of RU Lup. While no large flares are present, this variability is clearly of magnetic origin due to the corresponding plasma temperatures of around 30 MK; in contrast the cool plasma component at 2–3 MK is quite stable over a month, resulting in a drop of average plasma temperature from 35 MK down to 10 MK. Density analysis with the O vii triplet indicates high densities in the cool plasma, suggesting accretion shocks to be a significant contributor to the soft X-ray emission. No strong overall metal depletion is observed, with Ne being more abundant than Fe, that is at solar value, and especially O. Excess emission at 6.4 keV during the more active phase suggest the presence of iron fluorescence. Additionally RU Lup exhibits an extraordinary strong X-ray absorption, incompatible with estimates obtained at optical and UV wavelengths. Comparing spectra from a sample of main-sequence stars with those of accreting stars we find an excess of cool plasma as evidenced by lower O viii/O vii line ratios in all accreting stars. High density plasma appears to be only present in low-mass CTTS, while accreting stars with intermediate masses (2 M) have lower densities. Conclusions. In all investigated CTTS the characteristics of the cooler X-ray emitting plasma are influenced by the accretion process. We suspect different accretion rates and amounts of funnelling, possibly linked to stellar mass and radius, to be mainly responsible for the different properties of their cool plasma component. The exceptional X-ray absorption in RU Lup and other CTTS is probably related to the accretion flows and an optically transparent wind emanating from the star or the disk. Key words. stars: individual: RU Lupi – stars: pre-main sequence – stars: activity – stars: coronae – X-rays: stars 1. Introduction accreted from the stellar disk onto the star at almost free-fall ve- locity along magnetic field lines which disrupt the accretion disk T Tauri stars as a class, are very young low-mass stars. The mem- in the vicinity of the corotation radius. Upon impact, a strong bers of the subclass of so-called classical T Tauri stars (CTTS) shock is formed near the stellar surface and the funnelling of the are still accreting matter from a surrounding circumstellar disk. accreted matter by the magnetic field leads to “accretion spots" CTTS are thought to evolve first into weak-line T Tauri stars with filling factors at the percent level with respect to the stellar (WTTS), where they become virtually disk-less and no longer surface; therefore only a small fraction of the stellar surface is show signs of significant accretion, and eventually into solar- covered by the accretion spots. This accretion shock plasma is like star on the main sequence. Ongoing accretion in a CTTS expected to reach temperatures of up to a few MK and thus to is evidenced by an emission line spectrum and specifically, a lose a large fraction of its energy at UV and soft X-ray wave- large Hα equivalent width (EW > 10 Å). Further, the strong ob- lengths. The accreted plasma does not necessarily have to be at served infrared excess indicates the presence of a disk. In con- high density. However, if one wants to achieve the inferred small trast, WTTS have much weaker Hα emission and little or no filling factors at the percent level and the otherwise determined −8 −7 −1 IR-excess. The increasing dominance of the underlying contin- mass accretion rates of 10 –10 M yr , the infalling gas must uum emission over the optical stellar absorption line spectrum then be at rather high density (n > 1011 cm−3). The accretion gives rise to another sub-classification into moderate, veiled and process is accompanied by outflows from the star and possibly extreme T Tauri stars. However, these strict subdivisions are also from the disk, which probably also play an important role somewhat arbitrary and blurred since for example Hα emis- in the transport of angular momentum. Different mass accretion sion is highly variable and differs also within the CTTS class rates and filling factors, different stellar properties such as mass, by more than an order of magnitude. A detailed account of the rotation and activity and different viewing angles, naturally lead pre-main sequence stellar evolution of low-mass stars is given to the observed variety of the different CTTS phenomena. by Feigelson & Montmerle (1999). X-ray emission from T Tauri stars has already been detected In the commonly accepted magnetospheric accretion model with the Einstein and ROSAT observatories, and both types are for CTTS (Calvet & Gullbring 1998) material is assumed to be found to be copious and variable X-ray emitters. The origin of Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20077644 230 J. Robrade and J. H. M. M. Schmitt: X-rays from RU Lupi: accretion and winds in classical T Tauri stars the observed X-ray emission is currently the subject of some de- Table 1. Observing log of the XMM-Newton RU Lup exposures. bate. Since all main-sequence “cool stars”, i.e. stars with outer convection zones, are surrounded by X-ray emitting coronae Observation MOS (filt.) OM (Schmitt & Liefke 2004), some kind of magnetic activity is Date Time Dur.(ks) Exp.(no.) also expected for their pre-main sequence counterparts with their August 08 2005 04:19–12:54 30(28) 6 deep outer convection zones. Indeed, large sample studies such August 17 2005 09:17–17:02 27(15) 6 as COUP (Chandra Orion Ultradeep Project) showed that most Sept. 06/07 2005 18:30–02:16 27(22) 6 of their observed X-ray emission is of coronal origin (Preibisch et al. 2005). Further, magnetic fields, connecting star and disk, −1 have been invoked to explain the huge flares observed in CTTS both infalls and outflows with velocities up to 300–400 km s . (Favata et al. 2005a). Specifically, Lamzin (2000) attributes the origin of the observed On the other hand, the presence of accretion streams and lines profiles in the HST UV spectra to accretion shocks and outflows opens up additional possibilities for the generation of stellar winds. X-rays. Collimated winds have been observed as X-ray jets in From optical high-resolution spectra, Stempels & Piskunov ∼ Herbig-Haro objects (Pravdo et al. 2001; Favata et al. 2006) and (2002) determined mass and radius of RU Lup to M 0.8 M ∼ in CTTS such as DG Tau (Güdel et al. 2005). Accretion shocks and R 1.7 R. They further derived a projected rotational ve- = −1 are expected to generate plasma at a significantly higher density locity of v sin(i) 9kms and detected variability on short time scales of 1h. Together with various proposed periods (0.8– than stellar coronal plasma and their soft X-ray emission can ◦ ◦ be traced by density sensitive line ratios, e.g. between forbidden 3.7 d) this suggests an inclination between 3 and 16 ,i.e.a and intercombination lines in the He-like triplets of O vii and system that is viewed nearly pole-on with rather high intrinsic Ne ix. Anomalously low ratios of these lines have been observed rotation. Finally, they derived an interstellar absorption column = × 19 −2 for several CTTS, e.g. TW Hya (Kastner et al. 2002; Stelzer & towards RU Lup with a corresponding N(H) 6.3 10 cm . Schmitt 2004), BP Tau (Schmitt et al. 2005), CR Cha (Robrade Combining literature data with results derived from FUV data / & Schmitt 2006), V4046 Sgr (Günther et al. 2006) and MP Mus obtained with FUSE and HST STIS, Herczeg et al. (2005) es- (Argiroffi et al. 2007), and the inferred high plasma densities timated RU Lup to be an 0.6–0.7 M star with an age of ∼ × −8 suggest the presence of X-ray emission from shocks as postu- 2–3 Myr and a mass accretion rate of 5 10 M. Their de- = ± −2 lated in the magnetospheric accretion scenario. rived hydrogen column density of log N(H) 20.1 0.2cm , ∼ However, not all accreting stars seem to show high density corresponding to an extinction of AV 0.07, is again quite low.
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